Java – understanding integer Logic behind the implementation of highestonebit() method

The Java integer class has a static method, highestonebit method, which will return a value with a single bit, which is located in the highest bit of the specified value. If the specified value itself is equal to zero, it will return zero

For example, the input of int 17 will return 16; Because 17 can be expressed in binary as 10001, it will return the farthest bit left, equal to 16

In the integer class, it has the following implementation in Java doc

public static int highestOneBit(int i) {
    // HD,figure 3-1
    i |= (i >>  1);
    i |= (i >>  2);
    i |= (i >>  4);
    i |= (i >>  8);
    i |= (i >> 16);
    return i - (i >>> 1);
}

I just want to know the logic behind implementing it in this way and using the shift operation Anyone can know something about it

Solution

The algorithm calculates a given I, whose binary representation is:

0..01XXXXXXX...XXXX

value

0..011111111...1111

This is what the 5 | = operator does

Then, in the return statement, it subtracts the value shifted one bit to the right

0..001111111...1111

Get results

0..010000000...0000

How it works:

The highest possible bit is the 32nd (leftmost) bit Assume that the input number is 1 in this bit:

1XXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX

You or the value of this value shifts right by 1 (I > > 1) and gets

11XXXXXX XXXXXXXX XXXXXXXX XXXXXXXX

Then you or the value of the new value shifts right by 2 (I > > 2) and get

1111XXXX XXXXXXXX XXXXXXXX XXXXXXXX

Then you or the value of the new value shifts to the right by 4 (I > > 4) and get

11111111 XXXXXXXX XXXXXXXX XXXXXXXX

Then you or the new value of that value shifts right by 8 (I > > 8) and gets

11111111 11111111 XXXXXXXX XXXXXXXX

Finally, you or the new value of the value shifts to the right 16 (I > > 16) and gets

11111111 11111111 11111111 11111111

If the highest bit is less than the 32nd bit, these operations still change all bits to the right of it to 1 and keep the rest (higher bits) to 0